if you are using a factory mode (orig mod) you can never expect a good hashrate from your card, but if your card is already moded, i think you forgot to set each of your gpu in a compute mode, you can set it to your AMD Radeon directly.
Not a problem here. He was using 4GB card as video output here which is why speed dropped.
And if I got it correctly, the next options would be:
1) Put 8GB card (as I have some in another rig) in the 1st PCIE slot
2) Switch to Linux OS (HiveOS for example) - no experience there, so I leave that as last resort.
Any clue what would be the best use of the 4GB cards after Dec, when the DAG exceeds 4GB?
Yes, those 2 options would be your next hopes.
After DAG exceeds 4GB, you will have to go to other smaller minable coins and hope you have profit, otherwise selling them is the only way. I got rid of all my 4GB cards in beginning of year. Only have 1 left which will probably never mine again. Using it for Folding at Home.